Shah R. Valloppilly

Cluster synthesis and direct ordering of rare-earth transition-metal nanomagnets.

Rare-earth transition-metal (R-TM) alloys show superior permanent magnetic properties in the bulk, but the synthesis and application of R-TM nanoparticles remains a challenge due to the requirement of high-temperature annealing above about 800 °C for alloy formation and subsequent crystalline ordering. Here we report a single-step method to produce highly ordered R-TM nanoparticles such as YCo(5) and Y(2)Co(17), without high-temperature thermal annealing by employing a cluster-deposition system and investigate their structural and magnetic properties. The direct ordering is highly desirable to create and assemble R-TM nanoparticle building blocks for future permanent-magnet and other significant applications.

Magnetism of rapidly quenched rhombohedral Zr2Co11-based nanocomposites

The effect of quench rate and Zr content on nanostructure and magnetic properties of melt-spun ZrxCo100?x(x?=?16?21) is investigated. High quench rate favours the formation of rhombohedral Zr2Co11, which is the hard phase. The coercivity increases with an increase in quench rate. Zr addition in limited amounts decreases the grain size of magnetic phases, which may promote the effective exchange coupling of soft magnetic phases. Therefore, coercivity and maximum energy product of Zr2Co11-based materials are significantly enhanced. The best magnetic properties,iHc?=?3.0?kOe and (BH)max?=?4.6?MG?Oe, which are the highest reported values among Co?Zr binary alloys, are achieved for x?=?18. The temperature coefficients of coercivity and remanence between 100 and 380?K are ?0.05%?K?1, comparable to those of alnico magnet.

Coercivity Enhancement in

The Mo-content dependence of structure and magnetic properties of Zr₁₆Co_78-xMo_xSi₃B₃ ( x=0, 2, 3, 4, 5) nanocrystalline materials has been studied. The samples consist of hard-magnetic Zr₂Co₁₁ and soft-magnetic Co phases. The substitution of Mo for Co restrains the formation of Co, raises the content of Zr₂Co₁₁, and increases the mean grain size of Zr₂Co₁₁. Therefore, the coercive force of the sample increases with x. A coercive force of 7.9 kOe, which is a highest value reported among Zr-Co alloys, was achieved for x=5. The anisotropy field of Zr₂Co₁₁ remains almost unchanged with increasing Mo content.

{\rm Zr}_{2}{\rm Co}_{11}

The increasing interest in spin-based electronics has led to a vigorous search for new materials that can provide a high degree of spin polarization in electron transport. An ideal candidate would act as an insulator for one spin channel and a conductor or semiconductor for the opposite spin channel, corresponding to the respective cases of half-metallicity and spin-gapless semiconductivity. Our first-principle electronic-structure calculations indicate that the metallic Heusler compound Ti2MnAl becomes half-metallic and spin-gapless semiconducting if half of the Al atoms are replaced by Sn and In, respectively. These electronic structures are associated with structural transitions from the regular cubic Heusler structure to the inverted cubic Heusler structure.

-Based Nanocrystalline Materials Due to Mo Addition

Structural, electronic, and magnetic properties of a Heusler-type CoFeCrAl alloy have been investigated experimentally and by model calculations, with a focus on the alloys spin-gapless semiconductivity. The as-quenched samples are ferrimagnetic at room temperature with a Curie temperature of about 456 K, which increases to 540 K after vacuum annealing at 600 °C for 2 h. The saturation magnetizations of the as-quenched and 600 °C-annealed samples are 1.9 µB/f.u. and 2.1 µB/f.u., respectively, which are very close to the value predicted by the Slater–Pauling curve. The resistivity shows a nearly linear decrease with increasing temperature, from about 930 µΩ cm at 5 K to about 820 µΩ cm at 250 K, with dρ/dT of about −5 × 10−7 Ω cm K−1. We explain this high resistivity and its temperature dependence as imperfect spin-gapless semiconducting behavior, with a negative band-gap parameter of 0.2 eV.

Investigation of spin-gapless semiconductivity and half-metallicity in Ti2MnAl-based compounds

Die-upset Nd13.62Fe75.70Co4.45B5.76Ga0.47 magnets have been prepared with height reduction (h) in the range of 60 to 88%. The energy product as high as 50.4 MGOe was obtained in the sample with h ∼ 70%. The magnetic domains of the samples are revealed by using magnetic force microscopy (MFM). The average domain widths of the die-upset samples with surface normal parallel (//) and perpendicular (⊥) to the loading direction are in the range of w//: 0.4-0.6 μm; w⊥: 0.9-3.8 μm, respectively. These interaction domains are formed due to the strong inter-granular exchange interaction and magnetostatic interaction between grains. It was found that the ratio of φrms// to φrms⊥ is a good indicator for the quality of the magnet, where the φrms// and φrms⊥ are defined as the root-mean-square values of phase shift for the MFM images. The microstructures have been investigated by scanning electron microscopy (SEM), MFM and SEM results indicate the magnetic and crystalline microstructures are uniform for the sample with...

Magnetism, electron transport and effect of disorder in CoFeCrAl

Disordered CoFeCrAl and CoFeCrSi0.5Al0.5 alloys have been investigated experimentally and by first-principle calculations. The melt-spun and annealed samples all exhibit Heusler-type superlattice peaks, but the peak intensities indicate a substantial degree of B2-type chemical disorder. Si substitution reduces the degree of this disorder. Our theoretical analysis also considers several types of antisite disorder (Fe-Co, Fe-Cr, Co-Cr) in Y-ordered CoFeCrAl and partial substitution of Si for Al. The substitution transforms the spin-gapless semiconductor CoFeCrAl into a half-metallic ferrimagnet and increases the half-metallic band gap by 0.12 eV. Compared CoFeCrAl, the moment of CoFeCrSi0.5Al0.5 is predicted to increase from 2.01 μB to 2.50 μB per formula unit, in good agreement with experiment.

Magnetic micro-structural uniformity of die-upset Nd-Fe-B magnets

The structural, electronic, and magnetic properties of CoFeCrX (X = Si, Ge) Heusler alloys have been investigated. Experimentally, the alloys were synthesized in the cubic L21 structure with small disorder. The cubic phase of CoFeCrSi was found to be highly stable against heat treatment, but CoFeCrGe disintegrated into other new compounds when the temperature reached 402 °C (675 K). Although the first-principle calculation predicted the possibility of tetragonal phase in CoFeCrGe, the tetragonal phase could not be stabilized experimentally. Both CoFeCrSi and CoFeCrGe compounds showed ferrimagnetic spin order at room temperature and have Curie temperatures (TC) significantly above room temperature. The measured TC for CoFeCrSi is 790 K but that of CoFeCrGe could not be measured due to its dissociation into new compounds at 675 K. The saturation magnetizations of CoFeCrSi and CoFeCrGe are 2.82 μB/f.u. and 2.78 μB/f.u., respectively, which are close to the theoretically predicted value of 3 μB/f.u. for their...

Structural disorder and magnetism in the spin-gapless semiconductor CoFeCrAl

We report the fabrication of a rare-earth-free permanent-magnet material Co3Si in the form of nanoparticles and investigate its magnetic properties by experiments and density-functional theory (DFT). The DFT calculations show that bulk Co3Si has an easy-plane anisotropy with a high K1 ≈ −64 Merg/cm3 (−6.4 MJ/m3) and magnetic polarization of 9.2 kG (0.92 T). In spite of having a negative anisotropy that generally leads to negligibly low coercivities in bulk crystals, Co3Si nanoparticles exhibit high coercivities (17.4 kOe at 10 K and 4.3 kOe at 300 K). This result is a consequence of the unique nanostructure made possible by an effective easy-axis alignment in the cluster-deposition method and explained using micromagnetic analysis as a nanoscale phenomenon involving quantum-mechanical exchange interactions.

Magnetism and electronic structure of CoFeCrX (X = Si, Ge) Heusler alloys

The effect of Mo addition on phase composition and nanostructure of nanocrystalline Zr16Co84−xMox (x = 0–2.0) melt spun at 55 m/s has been investigated. All the ribbons consist mainly of a hard magnetic Zr2Co11 phase with rhombohedral crystal structure but also contain minor amounts of soft-magnetic phases. The increase in cell volume on alloying suggests that Mo mainly enters the rhombohedral Zr2Co11 structure and occupies the Co site. Mo addition promotes the formation of the hard magnetic phase and increases its volume fraction. The mean grain size of the hard magnetic phase remains almost unchanged with the increase of Mo content. But the average grain size of the soft magnetic phase decreases from about 200 nm to 50 nm. This promotes the exchange coupling of the hard and soft magnetic phases and thus leads to a significant increase in coercivity and isotropic energy product, from 0.6 kOe and 0.5 MGOe for x = 0 to 2.9 kOe and 4.2 MGOe for x = 1.5.